Lubricating composition for a four-stroke engine with low ash content

ABSTRACT

The present disclosure relates to a lubricating composition for a four-stroke engine, with low ash content including:
         a) one or more base oils selected from oils of Groups I-IV, preferentially from oils of Group III or IV from the API classification,   b) at least one compound (b) selected from the group of heavy PAOs with a kinematic viscosity at 100° C. comprised between 75 and 3,000 cSt, optionally mixed with one or more compounds selected from the group of polymeric compounds of the polyisobutene (PIB) type, or one of their mixtures, and   c) at least one ester of formula R(OH) m  (COOR′(OH) p ) n  wherein m is an integer from 0 to 8, preferably from 1 to 4, n is an integer from 1 to 8, preferably from 1 to 4 and p is an integer from 0 to 8, preferably from 1 to 4 and wherein the sum p+m is strictly greater than zero, R and R′ represent independently of each other a saturated or unsaturated, linear or branched hydrocarbon group, optionally substituted with one or more aromatic groups and including from 1 to 30 carbon atoms, and borated derivatives thereof;   the composition having a sulfated ash content less than or equal to 0.5%, as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm, as measured according to the ASTM D5185 standard, and a sulfur content less than 0.2%, as measured according to the ASTM D5185 standard.       

     The composition has good detergency properties, the use of which promotes fuel savings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Entry of International Application No. PCT/FR2008/001668, filed on Dec. 2, 2008, which claims priority to French Application 07 08 423, filed on Dec. 3, 2007, both of which are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a lubricating composition for a four-stroke engine with low ash content, having good detergency properties, the use of which promotes fuel savings, and comprising at least one hydroxylated or derived ester.

TECHNOLOGICAL BACKGROUND

Because of environmental concerns, it is increasingly sought to reduce polluting emissions and to achieve fuel savings in vehicles. The nature of the engine lubricant has an influence of both of these phenomena. On the one hand, the behavior of the lubricant in reducing friction will have an impact on fuel consumption. This is mainly the quality of lubricating bases, either alone or combined with polymers enhancing the viscosity index and with friction-modifying additives, which give to the lubricant its “fuel eco” properties.

On the other hand, because of the dilution of the fuel by a portion of the engine lubricant, certain components of the lubricants will be found after combustion in the exhausted gases of the vehicles. These components, in particular certain additives, may generate sulfur, phosphorus, sulfated ashes which will damage the post-treatment systems installed in vehicles. The ashes are detrimental for particle filters and phosphorus acts as a poison for catalytic systems.

In engine applications, it will therefore be sought to accordingly adapt the formulations of lubricants, whether this be in the selection of lubricant bases or additives. For automobile engines, the use of lubricants specifically formulated in observance of certain specifications, in particular the low ash content (so-called “low saps”) ACEA-C4 or (so-called “low saps” and “fuel eco”) ACEA-C1 specifications, elaborated by the European Automobile Manufacturers' Association, is preferred. These specifications impose to the lubricants limiting contents of sulfated ashes (generated by the presence of metals), sulfur and phosphorus, whence the designation “low saps” for “Sulfated Ashes, Phosphorus, Sulfur”.

Lowering the level of ashes, sulfur and phosphorus in engine lubricants while maintaining high levels of required performances is a challenge because these elements are present in the most currently used base oils and additives. Thus, for example, detergent additives, preventing the formation of deposits at the surfaces of metal parts by dissolution of secondary oxidation and combustion products, and which are key compounds of a formulation of an engine oil, contain metal salts which generate ashes. These are generally sulfonates, phenates, salicylates of earth alkaline metals such as calcium, magnesium, either overbased or not.

In order to be able to lower the detergent contents of the lubricants with the purpose of reducing its ash content, while maintaining adequate cleaning of the engine parts, it is necessary to act on the capacity of other compounds of the lubricant of forming a minimum of deposits. A possible route for obtaining low ash content lubricants, retaining good detergency behavior in spite of reduced detergent content, and having good fuel eco properties, is to prefer the use of certain unconventional mineral base oils or synthetic bases. These bases have the advantage of better temperature resistance and better resistance to oxidation as compared with conventional bases, which minimizes the formation of deposits. The treatment level of lubricants with detergent may thereby be lowered and the ash content lowered.

Their naturally high viscosity index (VI) leads to lubricants having good fuel eco properties, and the amount of polymers enhancing VI, generating deposits in the lubricants, may thereby be limited. Moreover, the sulfur content of unconventional mineral bases is comparatively lower than that of standard mineral bases because they undergo extensive hydrotreatments, and synthetic bases are sulfur-free, which facilitates observance of the limiting sulfur contents imposed by the ACEA specifications. By “unconventional mineral bases” are meant the bases of Group III according to the classification of the American Petroleum Institute (API), having a high viscosity index VI (typically above 130), commonly designated as “Group III+ bases”, or else the bases derived from GTL (Gas to Liquid) processes.

So-called Group III+ unconventional mineral bases are prepared in order to improve their cold properties, to decrease their volatility and increase their viscosity index VI), in order to reach values above 130. These are hydro-isomerized bases, which may be prepared from residues stemming from a hydrocracking treatment, possibly added with waxes or Gatsch petrolatum, and submitted to extensive catalytic deparaffinization. In the following description, and in the absence of any specification, the term “Group III bases” designates any type of mineral base of Group III according to the API classification, whereas the term “unconventional Group III base” or “Group III base” designates the Group III bases with VI above 130.

The synthetic bases are for example polyalphaolefins (PAOs), esters, and poly internal olefins (Group IV, V and VI, respectively, according to the API classification). In spite of their advantages, the aforementioned bases require a certain number of complex refining operations for the III+ Groups and of synthesis for Groups IV and V, which has an influence on their price and on their availability. The present invention relates to low saps and fuel eco lubricating compositions for a four-stroke engine, comprising an additivation system allowing the use of an increased number of lubricating bases, notably including conventional Group III bases.

SUMMARY

The object of the present invention is lubricating compositions for a four-stroke engine, containing additives providing per se a minimum of ashes, sulfur and phosphorus, totally or partly replacing standard additives used in this type of application, which in combination with each other, allow said lubricating compositions to retain their optimum detergency and fuel eco properties with low or very low content of ashes, sulfur, phosphorus. When one speaks here of “retaining” or “enhancing” the detergency properties of a lubricant, this notably means, at constant detergent content, the minimization of formation of deposits by the different components of the lubricant, notably high temperature deposits.

The compositions according to the invention are lubricating compositions for a four-stroke engine comprising:

a) one or more base oils selected from Group I-V oils, preferentially from Group III or IV oils, from the API classification.

b) at least one compound (b) selected from the group of heavy PAOs, with a kinematic viscosity at 100° C. comprised between 75 and 3,000 cSt; in an optional mixture with one or more compounds selected from the group of polymeric compounds of the polyisobutene (PIB) type, or one of their mixtures, and

c) at least one ester of formula R(OH)_(m) (COOR′(OH)_(p))_(n) wherein m is an integer from 0 to 8, preferably from 1 to 4, n is an integer from 1 to 8, preferably from 1 to 4, and p is an integer from 0 to 8, preferably from 1 to 4 and wherein the sum p+m is strictly greater than zero, R and R′ represent independently of each other a saturated or unsaturated, linear or branched, hydrocarbon group, optionally substituted with one or more aromatic groups and including from 1 to 30 carbon atoms, and its borated derivatives;

said composition having a sulfated ash content of less than or equal to 0.5%, as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm as measured according to the ASTM D5185 standard, and a sulfur content less than 0.2% as measured according to the ASTM D5185 standard.

According to a preferred embodiment, the ester (c) contains at least one free hydroxyl OH group belonging to the R group, said OH group being located in the alpha, beta or gamma position with respect to the carbon of the CO function of an ester function on which the group R is bound, and/or contains at least one free hydroxyl OH group belonging to the group R′, said OH group being located in the beta, gamma or delta position, with respect to the oxygen of the COO group of an ester function on which the group R′ is bound. Preferably, the group R′ of the ester (c) represents a C₁-C₁₀, preferably C₂-C₆ group. According to one embodiment, p is strictly greater than zero and the group R of the ester (c) represents a C₈-C₂₅, preferably C₁₂-C₁₈ group.

Preferably, at least one ester (c) will be selected from glycerol monoesters or diesters, preferentially selected from glycerol mono-oleate, glycerol stearate or isostearate and their borated derivatives. According to one embodiment, n is an integer comprised between 1 and 4 and the group R of the ester (c) represents a C₁-C₅, preferentially C₁-C₃ group. Preferably, at least one ester (c) is selected from citrates, tartrates, malates, lactates, mandelates, glycolates, hydroxypropionates, hydroxyglutarates or their borated derivatives.

The composition according to the invention leads to minimum fuel savings as measured on a M111FE test, of at least 2.5% and meets the ACEA-C1 specifications defined by the European Automobile Manufacturers' Association. According to a preferred embodiment, the composition has a kinematic viscosity at 100° C. as measured by the ASTM D445 standard comprised between 5.6 and 16.3 cSt, preferentially comprised between 9.3 and 12.5 cSt. According to a preferred embodiment, the composition has a grade of 5W30 according to the SAEJ300 classification. According to a preferred embodiment, the composition has a viscosity index VI larger than or equal to 130, preferentially larger than 150, preferentially larger than 160.

According to a preferred embodiment, in the composition, the base oil or the mixture of base oils (a) amounts to at least 70% by weight of said composition. According to a preferred embodiment, in the composition, the base oil or the mixture of base oils (a) comprises:

at least 60% by weight, based on the total weight of the lubricant, of one or more base oils of Group III

at least 10% by weight, based on the total weight of lubricant, of one or more base oils of Group IV.

According to a preferred embodiment, the composition comprises a compound (b) selected from the group of heavy PAOs, and a compound (b) selected from the group of polymeric compounds of the PIB type. According to a preferred embodiment, the composition comprises from 0.1 to 6%, preferentially from 2 to 4% of at least one compound (b) and from 0.1 to 2.5%, preferentially from 0.5 to 1.5% of at least one compound (c). According to a preferred embodiment, the composition comprises at least one anti-wear compound of the zinc dithiophosphate type, in possible combination with an amine phosphate. According to a preferred embodiment, the composition comprises at least one anti-wear compound of the zinc dithiophosphate type in an amount less than or equal to 1%, preferentially less than or equal to 0.5%.

According to a preferred embodiment, the composition is free of any additive of the amine phosphate type. According to a preferred embodiment, the composition is free of any molybdenum friction-modifying additive. According to a preferred embodiment, the composition comprises at least one antioxidant compound, preferentially ashless, preferentially of the phenolic or aminated type. According to a preferred embodiment, the composition comprises from 0.01 to 5% of one or more antioxidant additives.

According to a preferred embodiment, the composition has a BN, determined according to the ASTM D-2896 standard, less than or equal to 8 milligrams of potash per gram of lubricant, preferentially less than or equal to 6.5 milligrams of potash per gram of lubricant. According to a preferred embodiment, the composition comprises between 0 and 3%, preferably between 0 and 2.5% of a VI-enhancing polymer, selected from polymeric esters, olefin copolymers (OCPs), homopolymers or copolymers of styrene, butadiene or isoprene, polymethacrylates (PMAs).

According to another object, the invention relates to a method for making a composition according to the invention by diluting a package of additives comprising at least one compound (b) and at least one compound (c), in a base oil or a mixture of base oils (a), and wherein one VI-enhancing polymer is optionally added. According to a preferred embodiment, the package of additives is diluted so as to amount to 10-30%, preferentially 15-20% by weight of the lubricating composition and wherein the VI-enhancing polymer amounts to 0-3% by weight of the lubricating composition.

According to another object, the invention relates to a package of additives for a four-stroke engine lubricant with sulfated ash content less than or equal to 0.5%, as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm, as measured according to ASTM D5185, and a sulphur content less than 0.2% as measured according to the ASTM D5185 standard, characterized in that it comprises:

-   -   at least one compound (b), a heavy PAO optionally mixed with a         PIB;     -   at least one hydroxylated ester (c);     -   optionally, anti-wear and extreme pressure additives, friction         modifiers, detergents, antioxidants, detergents either overbased         or not, flow point lowering additives, dispersants, anti-foam         additives, thickeners, polymers enhancing the viscosity index.         Preferably, the package of additives comprises:     -   from 0.5-30% by weight, preferentially from 10-25% by weight, of         at least one compound (b), a heavy PAO optionally mixed with a         PIB;     -   from 0.5-15% by weight, preferentially from 2.75-8.75% by weight         of at least one hydroxylated ester (c).

According to another object, the invention relates to the use of a composition according to the invention as a lubricant for a four-stroke engine. According to another object, the invention relates to the use of at least one ester of formula R(OH)_(m) (COOR′(OH)_(p))_(n) wherein m is an integer from 0 to 8, preferably from 1 to 4, n is an integer from 1 to 8, preferably from 1 to 4 and p is an integer from 0 to 8, preferably from 1 to 4, and wherein the sum p+m is strictly greater than zero, R and R′ represent independently of each other a saturated or unsaturated, linear or branched hydrocarbon group, optionally substituted with one or more aromatic groups, and including from 1 to 30 carbon atoms, and its borated derivatives, as a friction modifying agent for the preparation of a lubricant composition for a four-stroke engine with a sulfated ash content less than or equal to 0.5% as measured according to the ASTM D874 standard, a phosphorus content less or equal to 500 ppm as measured according to ASTM D5185, and a sulfur content less than or equal to 0.2% as measured according to the ASTM D5185 standard.

DETAILED DESCRIPTION

The lubricating composition for a four-stroke engine according to the invention has a sulfated ash content less than or equal to 0.5% as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm as measured according to ASTM D5185, and a sulfur content less than 0.2% as measured according to the ASTM D5185 standard. Preferably, the compositions according to the present invention are of the ACEA C4 preferentially ACEA-C1 type, according to the specifications established by the European Automobile Manufacturers' Association for gasoline and diesel engine oils for lightweight vehicles.

The Table below groups together the sulfur, phosphorus, sulfated ash contents, and the performances in terms of fuel savings required by these specifications.

ACEA- ACEA- ACEA- ACEA- C1 C2 C3 C4 Maximum sulfated ash 0.5% 0.8% 0.8% 0.5% content in % (ASTM D874) Maximum sulfur content 0.2% 0.32%  0.3% 0.2% in % (ASTM D4294) Maximum phosphorus 500 ppm 900 ppm 900 ppm 500 ppm content (ASTM D5185) Minimum fuel savings 2.5% 2.5%   1%   1% (measured on a M111FE, CEC L54-T-96 engine test) According to an embodiment, the compositions according to the present invention have a sulfated ash content less than or equal to 0.30%, preferentially less than or equal to 0.25% and a phosphorus content less than or equal to 300 ppm, preferentially less than or equal 200 ppm.

1) Base Oils or Mixture of Base Oils (a)

The lubricating compositions according to the present invention comprise one or more base oils, generally amounting to at least 50% by weight of the lubricating compositions, generally larger than 70% and which may range right up to 90% and more.

The base oil(s) used in the compositions according to the present invention may be oils of mineral origin or synthetic oils of Groups I-V according to classes defined in the API classification (or their equivalents according to the ATIEL classification) as summarized below, either alone or in a mixture.

Saturated Sulfur ingredient content content Viscosity index Group I: Mineral oils <90% >0.03% 80 ≦ VI < 120 Group II: hydrocracked ≧90% ≦0.03% 80 ≦ VI < 120 oils Group III: hydrocracked ≧90% ≦0.03% ≧120 or hydro-isomerized oils Group IV PAOs: Polyalphaolefins Group V Esters and other bases not included in the bases of Groups I-IV These oils may be oils of vegetable, animal or mineral origin. Mineral base oils according to the invention include all types of bases obtained by atmospheric and in vacuo distillation of crude petroleum, followed by refining operations such as extraction by a solvent, deasphalting, dewaxing with a solvent, hydrotreating, hydrocracking and hydro-isomerization, hydrofinishing.

The base oils of the compositions according to the present invention may also be synthetic oils, such as certain esters of carboxylic acids and alcohols, or polyalphaolefins. The polyalphaolefins, used as base oils, and which are distinguished from heavy polyalphaolefins (b) also present in the compositions according to the present invention, are for example obtained from monomers having 4 to 32 carbon atoms (for example octene, decene) and a viscosity at 100° C. comprised between 1.5 and 15 cSt. Their weight average molecular weight is typically comprised between 250 and 3,000.

Mixtures of synthetic and mineral oils may also be used. There is no limitation to the use of such and such base oil for producing the compositions according to the present invention, if not that their amount and their nature should only be adjusted so as to obtain compositions having a sulfur content less than 0.2% as measured according to the ASTM D5185 standard, a viscosity grade and values of viscosity index or VI compatible with a use as a four-stroke engine oil. Preferably, the compositions according to the present invention have a kinematic viscosity at 100° C. comprised between 5.6 and 16.3 cSt as measured by the ASTM D445 standard, (grades SAE 20, 30 and 40), preferentially comprised between 9.3 and 12.5 cSt (grade 30). According to a particularly preferred embodiment, the compositions according to the present invention are of grade 5W30 according to the SAEJ300 classification. The compositions according to the present invention also preferably have a viscosity index VI larger than 130, preferentially larger than 150, preferentially larger than 160.

The main effect on the sulfur content of the lubricants is obtained by acting on the sulfur content of the base oils used: this is why base oils having a sulfur content less than 0.3%, for example mineral oils of Group III, and synthetic sulfur-free bases, preferentially of Group IV, or a mixture thereof, may be used advantageously. Thus, the compositions according to the present invention may contain at lest 70% of base oil, typically at least 60% by weight of one or more base oils of Group III, and at least 10% by weight of one or more base oils of Group IV.

2) The Compounds (b): “Heavy” Polyalphaolefins (PAOs) or Polyisobutenes (PIBs)

The compounds (b) of the “heavy” polyalpha olefin (PAO) type or polyalpha olefins “with viscosity” entering the composition according to the invention are selected from PAOs with a kinematic viscosity at 100° C. measured according to ASTM D445, comprised between 75 and 3,000 cSt, preferentially comprised between 150 and 1,500, preferentially between 300 and 1,200 cSt. Their number average molecular weight Mn is preferentially larger than 2,500, typically comprised between 3,000 and 20,000, preferentially between 3,000 and 10,000, preferentially between 3,000 and 7,000. Their weight average molecular weight Mw is typically of the order of approximately 4,000 to 50,000, and their polydispersity index Mw/Mn is of the order of 1.1 to 5 and more.

These polyalphaolefins are for example obtained from monomers such as octene, decene, dodecene, tetradecene, hexadecene, etc., either alone or mixed with other olefins. They may be used alone or mixed in compositions according to the invention.

The compounds (b) of the polyisobutene (PIB) type entering the composition according to the invention are liquid polymeric compounds soluble in oil. Their weight molecular weight Mw is typically larger than 800, typically comprised between 800 and 8,000, usually between 1,500 and 7,000. Their kinematic viscosity at 100° C. is preferentially comprised between 1,000 and 6,000 cSt (ASTM D445). Typically, the PIBs according to the present invention have a weight average molecular weight comprised between 2,000 and 5,000 and a kinematic viscosity at 100° C. comprised between 3,000 and 4,500 cSt.

The composition may comprise at least one compound selected from the group of heavy PAOs described above, optionally mixed with at least one compound selected from the group of polymeric compounds of the PIB type described above. These compounds (b), either alone or in a mixture, are totally or partially substituted for VI-enhancing polymer generating deposits usually present in engine lubricants formulated with conventional bases. They therefore allow the detergent content to be lowered. However, fuel eco performances are a little degraded, because of their bad cold behavior.

3) Hydroxylated Esters (c)

The composition further comprises at least one ester of formula R(OH)_(m) (COOR′(OH)_(p))_(n) wherein m is an integer from 0 to 8, preferably from 1 to 4, n is an integer from 1 to 8, preferably from 1 to 4, and p is an integer from 0 to 8, preferably from 1 to 4 and wherein the sum p+m is strictly greater than zero, R and R′ represent independently of each other a saturated or unsaturated, linear or branched hydrocarbon group, optionally substituted with one or more aromatic groups and including from 1 to 30 carbon atoms, and its borated derivatives. According to a preferred embodiment, the ester (c) has at least one free hydroxyl OH group belonging to the group R, said OH group being located in the alpha, beta or gamma position with respect to the carbon of the CO function of an ester function on which the group R is bound, and/or contains at least one free hydroxyl OH group belonging to the group R′, said OH group being located in the beta, gamma or delta position with respect to the oxygen of the COO group of an ester function on which the group R′ is bound.

Preferably R′ represents a C₁-C₁₀, preferably C₂-C₆ group. Preferably R represents a C₈-C₂₅, preferably C₁₂-C₁₈ group. According to one embodiment, p is strictly greater than zero when the group R of the ester (c) represents a C₈-C₂₅, preferably C₁₂-C₁₈ group. According to another embodiment, n is an integer comprised between 1 and 4 when the group R of the ester (c) represents a C₁-C₅, preferentially C₁-C₃ group.

The hydroxylated esters (c) may be selected from monoesters or diesters obtained from glycerol such as glycerol mono-oleate, glycerol stearate or isostearate and their borated derivatives. The hydroxylated esters (c) may also be selected from citrates, tartrates, malates, lactates, mandelates, glycolates, hydroxypropionates, hydroxyglutarates or their borated derivatives.

Surprisingly, the Applicant has shown that by using the compounds (c), in combination with the compounds (b), it was possible to formulate from conventional bases, engine lubricants with good fuel eco performances and good detergency properties, even if the detergent content remains moderate. Fuel eco, low saps engine lubricants retaining good detergency properties are thereby obtained. The compositions according to the invention may for example comprise from 0.1 to 6%, preferentially from 2 to 4% of at least one compound (b) and from 0.1 to 2.5%, preferentially from 0.5 to 1.5% of at least one compound (c). The esters of formula R(OH)_(m) (COOR′(OH)_(p)), according to the invention are prepared according to methods known to one skilled in the art, notably by reacting a carboxylic acid of formula R(OH)_(m) (COOH)_(n) with an alcohol of formula R′(OH)_(p), the substituents R, R′ and the indices m, n being as defined above.

4) Other Additives

The compositions according to the invention may further contain any type of suitable additives for use as four-stroke engine oil. These additives may be introduced separately and/or included in additive packages used in the formulations of commercial lubricants for 4-stroke engines, with performance levels as defined by the ACEA (European Automobile Manufacturers' Association) and/or the API (American Petroleum Institute) well known to one skilled in the art. Thus, the compositions according to the invention may notably contain in a non-limiting way, anti-wear and extreme pressure additives, friction modifiers, antioxidants, detergents either overbased or not, polymers enhancing the viscosity index, flow point enhancers, dispersants, anti-foam agents, thickeners . . . .

The anti-wear and extreme pressure additives protect the rubbing surfaces by forming a protective film adsorbed on these surfaces. The most currently used one is zinc dithiophosphate or DTPZn. Various phosphorus, sulfur, nitrogen, chlorine and boron compounds are also found in this category. There exists a large diversity of anti-wear additives, but the category which is the most used in engine oils is that of the phosphorus-sulfur additives such as metal alkylthiophosphates, in particular zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates or DTPZn. The preferred compounds are of formula Zn((SP(S)(OR1)(OR2))2, wherein R1 and R2 are alkyl groups, preferentially including from 1 to 18 carbon atoms. DTPZn is typically present in levels of the order of 0.1-2% by weight in the engine oils.

Amine phosphates are also anti-wear additives currently used. However, phosphorus provided by these additives, acts as a poison for catalytic systems of automobiles, and they also provide ashes. These effects may be minimized by partly replacing them with additives not providing phosphorus, such as for example polysulfides, notably sulfur-containing olefins.

In lubricating compositions, anti-wear and extreme pressure additives of the nitrogen- and sulfur-containing type such as for example metal dithiocarbamates, in particular molybdenum dithiocarbamate, which also are generators of ashes, are also usually encountered. Glycerol esters of are also anti-wear additives. For example, mono-, di- and tri-oleates, monopalmitates and monomyristates may be mentioned. The anti-wear and extreme pressure additives are present in the compositions for engine lubricants in levels comprised between 0.01 and 6%, preferentially comprised between 0.01 and 4%.

In the compositions according to the invention, by the presence of hydroxylated esters or polyalcohol esters (c) it is possible to limit the amounts of sulfur-containing, phosphorus/sulfur-containing, nitrogen-containing, and sulfur-containing additives and of phosphates, so as to achieve a low content of sulfated ash, sulfur and phosphorus, for example compatible with the ACEA-C and ACEA-C2 specifications, while retaining performances compatible with a use as a four-stroke engine oil. The lubricating compositions according to the present invention may contain DTPZn, and/or other anti-wear and extreme pressure additives, in amounts compatible with an overall sulfated ash content less than 0.5% as measured according to the ASTM D874, a phosphorus content less than 500 ppm as measured according to ASTM D5185, and a sulfur content less than 0.2% as measured according to ASTM D5185 standard. They may contain a DTPZn content less than or equal to 1% by weight, preferably less than or equal to 0.5%. They may also be free (0% by weight) of additives providing phosphorus, for example amine phosphate.

The friction modifiers encountered in lubricating compositions for a four-stroke engine may be compounds providing metal elements or else ashless compounds. Solid compounds are also found, such as molybdenum sulfide, graphite or PTFE. The metal compounds for example are complexes of transition metals such as Mo, Sb, Sn, Fe, Cu, Zn, the ligands of which may be hydrocarbon compounds containing oxygen, nitrogen, sulfur or phosphorus atoms. In particular, molybdenum-containing compounds may be particularly effective, such as for example molybdenum dithiocarbamates, dithiophosphates.

The ashless friction modifiers may for example be fatty alcohols, fatty acids, esters, fatty amines. The friction modifying additives are generally present in levels comprised between 0.01 and 5%, preferentially 0.01 and 1.5% in the engine lubricants.

In the compositions according to the invention, by the presence of hydroxylated esters or polyalcohol esters (c) it is possible to limit the amount of friction modifiers, providers of sulfated ashes, phosphorus and sulfur, so as to reach a low content of sulfated ash, sulfur and phosphorus, for example compatible with the ACEA-C1 and ACEA-C4 specifications, while retaining performances compatible with use as a four-stroke engine oil, notably fuel eco or fuel saving properties, so as to be included in the ACEA-C1 specification. The compositions according to the present invention may be free of friction modifiers providing ashes, for example molybdenum friction modifiers. The lubricating compositions according to the present invention may however contain any friction modifying additive types, in amounts compatible with an overall content of sulfated ashes less than or equal to 0.5% as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm as measured according to ASTM D5185, and a sulfur content less than or equal to 0.2% as measured according to the ASTM D5185 standard.

The antioxidants delay the degradation of oils in use, a degradation which may be expressed by the formation of deposits, the presence of sludges, or an increase in the viscosity of the oil. They act as radical inhibitors or hydroperoxide-destroying agents. Aminated antioxidants of the phenolic type are found among the antioxidants currently used. Some of these additives, for example the phosphorus-sulfur additives, may be generators of ashes.

Phenolic antioxidants may be ashless, or else be in the form of neutral or basic metal salts. Typically, these are compounds containing a sterically hindered hydroxyl group, for example when 2 hydroxyl groups are in the ortho or para position with respect to each other, or when the phenol is substituted with an alkyl group including at last 6 carbon atoms. The aminated compounds are another class of antioxidants which may be used, possibly in combination with phenolic compounds. Typical examples are the aromatic amines, of formula R₈R₉R₁₀N, wherein R₈ is an aliphatic group or an optionally substituted aromatic group, R₉ is an optionally substituted aromatic group, R₁₀ is hydrogen, or an alkyl or aryl group or a group of formula R₁₁S(O)_(x)R₁₂, wherein R₁₁ is an alkylene, alkenylene, or aralkylene group, and x is equal to 0, 1 or 2.

Sulfurized alkylphenols or their alkaline and earth alkaline salts are also used as antioxidants. Another class of antioxidants is that of the copper compounds soluble in oil, for example copper thio- or dithio-phosphates, copper salts of carboxylic acids, copper dithiocarbamates, sulfonates, phenates, acetylacetonates. Copper(I) and (II) salts of succinic acid or anhydride are used. These compounds, either alone or in a mixture, are typically present in the lubricating compositions for a 4-stroke engine in amounts comprised between 0.1 and 5% by weight. The lubricating compositions according to the present invention may contain all types of antioxidant additives known to one skilled in the art, in amounts compatible with an overall content of sulfated ashes less than or equal to 0.5% as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm, as measured according to ASTM D5185, and a sulfur content less than or equal to 0.2% as measured according to the ASTM D5185 standard. Ashless antioxidants will be preferred.

The detergents reduce the formation of deposits at the surface of the metal parts by dissolving secondary oxidation and combustion products. The detergents used in the lubricating compositions according to the present invention are well known to one skilled in the art. The detergents commonly used in the formulation of lubricating compositions are typically anionic compound including a long lipophilic hydrocarbon chain and a hydrophilic head. The associated cation is typically a metal cation of an alkaline or earth alkaline metal.

The detergents are preferentially selected from alkaline and earth alkaline metal salts of carboxylic acids, sulfonates, salicylates, naphthenates, as well as phenate salts. The earth alkaline and alkaline metals are preferentially calcium, magnesium, sodium or barium. These metal salts may contain the metal in an approximately stoichiometric amount or else in excess (in an amount above the stoichiometric amount). In the latter case, one is dealing with so-called overbased detergents.

The excess metal providing the overbased character to the detergents exists as metal salts insoluble in oil, for example a carbonate, hydroxide, oxalate, acetate, glutamate, preferentially a carbonate. In a same overbased detergent, the metals of these insoluble salts may be the same of those of the detergents soluble in oil or else be different. They are preferentially selected from calcium, magnesium, sodium or barium.

The overbased detergents thus appear as micelles consisting of insoluble metal salts held in suspension in the lubricating composition by detergents as soluble metal salts in oil. These micelles may contain one or more types of insoluble metal salts stabilized by one or more detergent types. The overbased detergents including a single type of detergent-soluble metal salts will generally be named from the nature of the hydrophobic chain of the latter detergent. Thus, they will be said to be of the phenate, salicylate, sulfonate, naphthenate type depending on whether this detergent is a salicylate, sulfonate or naphthenate, respectively.

The overbased detergent will be said to be of the mixed type if the micelles comprise several types of detergents, different from each other by the nature of their hydrophobic chain. The lubricating compositions according to the present invention may contain all types of detergents known to one skilled in the art, either neutral or else overbased or else strongly overbased.

The more or less overbased character of the detergents is characterized by the BN (base number), measured according to the ASTM D2896 standard, and expressed in mg of KOH per gram. Neutral overbased detergents have a BN comprised between about 0 and 80. The overbased detergents themselves have BN values typically of the order of 150 and more, or even 250 or 450 or more. The BN of the lubricating composition containing the detergents is measured according to the ASTM D2896 standard and is expressed in mg of KOH per gram of lubricant.

It is generally desirable to include in the lubricating compositions for engines, at least one portion of detergents in an overbased form, so as to allow neutralization of certain acid impurities stemming from the combustion and found in the oil. This being the case, these compounds contain metal salts which generate ashes, and the formulation of lubricants with low ash content according to the present invention imposes, while maintaining good detergency properties, adjustment of the amount of notably overbased detergents. Thus, the lubricating compositions according to the present invention may contain any type of detergents known to one skilled in the art, either neutral or else overbased, or else strongly overbased, in amounts compatible with an overall content of sulfated ashes less than or equal to 0.5% as measured according to the ASTM D2896 standard. Preferentially, the amounts of overbased detergents included in the lubricating compositions according to the invention, are adjusted so that the BN of said compositions, as measured according to the ASTM D2896 standard, is less than or equal to of 8 mg of KOH per gram of lubricant, preferentially less than or equal to 6.5, preferentially comprised between 3 and 6.

With the viscosity-enhancing polymers, good cold strength may be guaranteed as well as minimum viscosity at high temperatures, notably for formulating multigrade oils. By introducing these compounds into the lubricating compositions, viscosity index (VI) values may be attained which give them good fuel eco or fuel-saving properties. Thus, the lubricating compositions according to the invention have preferably VI values, as measured according to ASTM D2270, larger than or equal to 130, preferentially larger than 150, preferentially larger than 160.

For example, among these compounds, mention may be made of polymeric esters, olefin copolymers (OCPs), homopolymers or copolymers of styrene, butadiene or isoprene, polymethacrylates (PMAs). They are conventionally present at levels of the order of 0 to 40° A), preferentially from 0.01 to 15% by weight, in the lubricating compositions for a four-stroke engine. These compounds however have the drawback of forming deposits and their presence in the formulations, notably in fuel eco formulations, leads one skilled in the art to increasing the detergent content in the lubricants, which generates ashes and with which both low saps and fuel eco specifications of the ACEA Cl or ACEA C2 type cannot be met. In the lubricants according to the present invention, the presence of heavy PAOs (b) in an optional mixture with a PIB (b), as a total or partial replacement of VI-enhancing polymers, and in combination with the hydroxylated esters (c), allows the detergent treatment levels to be reduced, and therefore a low ash content may be achieved. Low sap engine lubricants are thereby obtained with fuel-saving properties and non-degraded detergency performances.

The lubricating compositions according to the present invention may contain of the order of 0.0 to 10% by weight of VI-enhancing polymers. Preferably, the compositions according to the invention contain at most 3% by weight of VI-enhancing polymers which generate deposits, for example selected from polymeric esters, olefin copolymers (OCPs), homopolymers or copolymers of styrene, butadiene or isoprene, polymethacrylates (PMAs), preferentially at most 2.5% by weight, or are free of them.

Flow point lowering additives enhance the cold behavior of the oils, by slowing down the formation of paraffin crystals. For example, these are alkyl polymethacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes, alkylated polystyrene . . . . The dispersants such as for example succinimides, PIB (polyisobutene) succinimides, Mannich bases ensure that the insoluble solid contaminants formed by the secondary oxidation products which are formed when the engine oil is being used, are held in suspension and evacuated.

In order to prepare the lubricating compositions according to the invention, the compound (b), heavy PAOs and/or PIB, hydroxylated ester(s) (c) may be introduced as individual compounds. All or part of the additives may be a part of a concentrate or package of additives which will be diluted in the base oil or in the mixture of base oils (a). Thus, some additives may be introduced via the package and other ones individually. The VI-enhancing polymers may in particular be added independently of the package. The object of the present invention is also such preparation methods, in particular a method wherein the package of additives is diluted so as to amount to 10-30%, preferentially 15-20% by weight of the lubricating composition, and wherein the VI-enhancing polymer amounts to 0-3% of the lubricating composition.

The object of the present invention is also packages of additives for a four-stroke engine lubricant with a sulfated ash content less than or equal to 0.5% as measured according to the ASTM D874 standard, a phosphorus content is less than or equal to 500 ppm as measured to ASTM D5185, and a sulfur content is less than or equal to 0.2% as measured according to the ASTM D5185 standard, which comprise:

-   -   at least one compound (b), a heavy PAO optionally mixed with a         PIB     -   at one hydroxylated ester (c)     -   optionally, anti-wear and extreme pressure additives, friction         modifiers, detergents, antioxidants, detergents either overbased         or not, flow point lowering additives, dispersants, anti-foam         additives, thickeners, polymers enhancing the viscosity index.

Preferentially, the packages of additives according to the present invention comprise:

-   -   from 0.5 to 30% by weight, preferentially from 10 to 25% by         weight, of at least one compound (b), a heavy PAO optionally         mixed with a PIB     -   from 0.5 to 15% by weight, preferentially from 2.75 to 8.75% by         weight of at least one hydroxylated ester (c).

Another object of the present invention relates to the use of lubricating compositions as described above, as a lubricant for four-stroke either diesel or gasoline engines, preferentially for lightweight vehicle engines. Finally, the present invention relates to the use of hydroxylated esters (c) as described above, as friction modifiers with which lubricating compositions for four-stroke engines may be formulated with a sulfated ash content less than or equal to 0.5%, as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm as measured according to ASTM D5185, and a sulfur content less than or equal to 0.2% as measured according to ASTM D5185 standard. Of course, the present invention is not limited to the described and illustrated examples and embodiment, but it is open to many alternatives accessible to one skilled in the art.

Example 1 Improvement of the Detergency Properties by Means of PIBs (b′) and Heavy PAOs (b)

Description of the Compositions and Preparation

The compositions are described in Table 1.

The composition A is a reference composition of 5W30 grade comprising a VI-enhancing polymer of the OCP type.

In the compositions B, C and D, a portion of the VI-enhancing OCP polymer was substituted with PIBs or the heavy PAO, with respect to reference A.

TABLE 1 Composition and properties A B C D Base oil Gr IV, mass % 15.00 14.98 14.48 15.13 Base oil Gr III, 22..84 22.81 22.05 23.04 KV 100° C. = 5 cSt, mass % Base oil Gr III, 40.00 39.95 38.62 40.36 KV 100° C. = 4.3 cSt, mass % VI-enhancing OCP polymer, 8.70 7.30 6.88 4.00 mass % Heavy PAO, 4.00 KV 100° C. = 1,000 mm²/s, (Spectrasyn Ultra 1000 marketed by Exxon Mobil), mass % PIB (Indopol H2100, marketed by 1.50 Innovene-Ineos Oligomers), mass % PIB (Indopol H100, marketed by 4.50 Innovene-Ineos Oligomers), mass % Phosphorus, ppm, ASTM D5185 500 500 500 500 Sulfur, ppm, ASTM D5185 1800 1800 1800 1800 Sulfated ashes, mass % 0.5 0.5 0,.5 0.5 TBN (mg KOH/g) ASTM D2896 4.6 4.6 4.6 4.6 KV 100 mm²/s, ASTM D445 12 11.94 11.97

In order to investigate the influence of substituting for the VI-enhancing OCP polymer, heavy PAOs and PIBs (compounds b and b′ according to the invention, respectively), two laboratory detergency tests were carried out on oils A, B, C, and D.

MCT Microcoking Test

MCT (Micro Coking Test) is a test for evaluating the deposit-forming tendency on a hot surface (coking).

The test conditions are the following:

MCT (according to the GFC Lu-27-A-03 v.2 standard)

-   -   60 μL of oil (+10 ppm of anti-foaming agent)     -   duration: 90 min     -   plate tilted by 1-2% including a bucket     -   a temperature gradient from 230 to 280° C.     -   scoring of the varnishes of the plate: so-called “division of         squares (/10)” method 2.

ECBT Test

ECBT (Elf Coking Bench Test) is a test of heat strength on a coking bench. It simulates an engine piston brought to a high temperature which is spread with oil projections from a crankcase. The lubricant arrives on a very hot surface, is altered and gives rise to deposits. With this test, it is possible to approach the nature of the deposits formed on the upper portions of the piston (crown, first groove and piston bottom) as well as obtain an estimation of the resistance of the oil to alteration. As compared with MCT, this test is dynamic. A description of this test may be consulted in the Proceedings of the Motorship Marine Propulsion Conference 2000, Amsterdam, March 29-30, “The relevance of laboratory tests in simulating field performance”, by J P. Roman.

ECBT Test Conditions

About 400 g of oil.

Duration: 1 hr sweep at different temperatures

Temperatures: 290, 300 and 310° C.

Aluminum beaker

No final sweep during cooling

Scoring of the sweeped area of the aluminum beaker (/100)

F9Q Detergency Engine Test

Detergency properties are also evaluated by an F9Q engine test, under the following conditions:

-   -   1.9 liter common rail diesel engine     -   duration 96 hours     -   4,000 rpm at full load     -   scoring of piston fouling (varnish/carbon/global)

Table 2 gives the results of the detergency tests obtained for oils A, B, C and D. It is seen that at a set detergent content (a set TBN), detergency performances of oils B, C, D where PIBs or heavy PAO are partially substituted for the OCP polymer, are better than those of the reference oil A, which only contains OCP.

TABLE 2 detergency tests Heavy REF PIB PIB PAO A B C D Laboratory detergency tests MCT (av. merit/10) Score 2 8.0 8.5 8.5 8.6 ECBT, 1 hr@290° C. (score/100) 34.3 50.4 38 36.6 ECBT, 1 hr@300° C. 19.5 32.4 23.9 22.5 ECBT, 1 hr@310° C. 16 22.5 22.9 16.9 F9Q detergency engine test Carbon score/10 8.2 8.4 Varnish score/10 4.9 5.3 Overall score/100 59.4 62.2

Example 2 Improvement of Fuel Eco Properties by Means of FM Triethyl Citrate (c) in the Presence of PIB (b)

Description of the Compositions and Preparation

Composition A′ is a reference composition with a very low ash, sulfur and phosphorus content, of 5W30 grade, with a VI-enhancing polymer different from that of reference A. The mass composition of the A′ oil as well as its properties, are given in Table 2. The composition B′ was prepared by adding 1% by mass of triethyl citrate to the composition A′.

Fuel Eco Properties

The fuel eco properties of oils A′ and B′ were measured by a laboratory friction test Cameron Plint. This laboratory test is correlated with the M111FE (CEC L54-T-96 standard) engine tests. The test bench consists of a plane cylinder tribometer immersed in the oil to be tested. A variable normal force is applied on the heated plane and the resulting friction force is measured. By comparing the results with those obtained for the reference oils from the Mill FE (CEC L54-T-96 standard) engine test, it is possible to calculate the fuel savings made with the tested oil.

The results reported in Table 3 below show the improvement in fuel eco properties induced by addition of triethyl citrate.

TABLE 3 A′ B′ Gr III, KV 100° C. = 4 cSt, mass % 50.00 49.50 Gr III, KV 100° C. = 6 cSt, mass % 20.00 19.80 Base oil Gr IV, PAO KV 100° C. = 4 cSt, 10.00 9.90 mass % VI-enhancing polymer (hydrogenated 6.50 6.43 isoprene-styrene), mass % Triethyl citrate (a), mass % — 0.99 PIB (b), mass % 2.00 1.98 Phosphorus, ppm 183 181 Sulfur, mass % 0.053 0.052 Sulfated ashes, mass % 0.23 0.23 TBN (mg KOH/g) ASTM D2896 3.43 3.40 TBN (mg KOH/g) ASTM D4739 2.53 2.50 KV 100 mm²/s 11.89 11.89 Cameron Plint (fuel savings %) 1.75 2.02

Example 3 Improvement of Fuel Eco Properties and Detergency by the Heavy PAO (b)+Triethyl Citrate (c) Combination and by the Heavy PAO (b)+Glycerol Monoisostearate (c) Combination

Description of the Compositions and Preparation

Composition E is a reference composition of grade 5W30 comprising a VI-enhancing OCP type polymer, and a package of additives comprising dispersants, detergents (weakly and strongly overbased calcium sulfonates and phenates), DTPZn, a friction modifier, aminated and phenolic antioxidants, an anti-foam additive, a flow point lowering agent.

In compositions F, G and H, as compared with reference E, a portion of the VI-enhancing OCP polymer is substituted with a heavy PAO, with a kinematic viscosity at 100° C. of 1,000 mm²/s, marketed by Exxon Mobil under the name of SpectraSyn Ultra 1000.

Compositions G and H are compositions according to the invention, wherein the VI-enhancing polymer is partly substituted with this same heavy PAO (compound (b)) and further comprising 1% by weight of a compound (c), triethyl citrate and glycerol monoisostearate, respectively.

The compositions (in mass %) and the physicochemical properties of compositions E, F, G, H, are given in Table 4.

“Fuel Eco” or Fuel-Saving Properties

“Fuel eco” or fuel-saving properties of the compositions E, F, G, H were evaluated by a M111FE engine test and by a Cameron Plint laboratory test.

The detergency properties were evaluated by the ECBT test conducted at 280° C.

The results are grouped in Table 5.

Conditions of the Fuel Eco Engine Test M111FE (CEC L54-T-96 Standard)

2 L gasoline engine of 100 kW

Cycles which represent haulage with strong urban character:

-   -   engine speed between 750 and 3,070 rpm     -   power between 0 and 49 KW     -   oil temperature between 20 and 75° C.

Measured fuel savings as compared with a reference oil of grade 15W40 (RL191).

Conditions of the Cameron Plint Fuel Eco Laboratory Friction Test

This laboratory test is correlated with M111FE (CEC L54-T-96 standard) engine tests. The test bench consists of a plane cylinder tribometer, immersed in the oil to be tested. A variable normal force is applied on the heated plane and the resulting friction force is measured. By comparing the results with those obtained for the reference oils from the M111 FE (CEC L54-T-96 standard) engine test, it is possible to calculate the fuel savings made with the tested oil.

TABLE 4 compositions (in mass %) and physicochemical properties Base oil: E (Ref) F G H Gr IV PAO, KV 100° C. = 6 cSt 15.20% 15.20% 15.05% 15.05% Gr III, KV 100° C. = 6 cSt 14.90% 14.80% 14.65% 14.65% Gr III KV 100° C. = 4 cSt 50.80% 50.80% 50.30% 50.30% Total base oil 80.90% 80.80% 80.00% 80.00% OCP polymer  5.10%  2.20%  2.20%  2.20% Heavy PAOs, KV at 100° C. 1000 mm2/s,  3.00%  3.00%  3.00% Spectrasyn 1000 Glycerol monoisostearate  1.00% Triethyl citrate  1.00% Additive package comprising: weakly and 13.80% 13.80% 13.80% 13.80% strongly strongly overbased detergents; aminated and  0.50%  0.50%  0.50%  0.50% phenolic ashless antioxidants, succinimide dispersant, amine phosphate anti-wear agent, including, DTPZn Flow point lowering agent  0.20%  0.20%  0.20%  0.20% Sulfated ashes*  0.50%  0.50%  0.50%  0.50% Phosphorus** (ppm) 509 ppm 509 ppm 500 ppm 500 ppm % sulfur*** 0.128% 0.128% 0.128% 0.128% TBN mgKOH/g ASTM D2896 5.95 5.95 5.95 5.95 KV at 100° C., in mm²/s ASTM D445 9.96 9.94 10.04 9.952 KV at 40° C., in mm²/s ASTM D445 57.65 57.81 57.99 56.64 VI 160 159 161 164 *ASTM D874 measurement, **ASTM D5185 measurement, ***ASTM D5185 measurement.

TABLE 5 fuel eco and detergency properties Compositions E (ref) F G H Detergency ECBT 280° C. 25.90 34.20 36.90 35.30 Fuel Economy FE Cameron Plint 1.97 1.78 2.38 2.04 (fuel savings %) M 111 FE 2.57 1.90 2.19 2.50 (fuel savings %)

Detergency Results

By partly substituting OCP with heavy PAO, in composition F, the formation of deposits may be minimized, therefore detergency properties may be improved as compared with reference E, with a constant detergent treatment level. Addition of hydroxylated esters has no influence and the good detergency performances are preserved.

Fuel Eco Results

Partial substitution of the VI-enhancing OCP type polymer with a heavy PAO has a positive effect on detergency (transition from 25.30 to 34.20 in ECBT 280° C.) but a negative effect on fuel eco properties (transition from 1.97 to 1.78 in the Cameron Plint test and from 2.57 to 1.90 in the M111FE engine test). By adding hydroxylated ester in compositions G and H, it is possible to compensate lowering of the fuel eco property and to even improve it relatively to the reference. 

1. A lubricating composition with low ash content for four-stroke engines comprising: a) one or more base oils selected from the oils of Groups I-V, preferentially from oils of Group III or IV from the API classification; b) at least one compound (b) selected from the group of heavy PAOs with a kinematic viscosity at 100° C. comprised between 75 and 3,000 cSt, in an optional mixture with one or more compounds selected from the group of polymeric compounds of the polyisobutene (PIB) type; and c) at least one ester of formula R(OH)_(m) (COOR′(OH)_(p))_(n) wherein m is an integer from 0 to 8, n is an integer from 1 to 8, and p is an integer from 0 to 8, and wherein the sum p+m is strictly greater than zero, R and R′ represent independently of each other a saturated or unsaturated, linear or branched hydrocarbon group, optionally substituted with one or more aromatic groups, and including from 1 to 30 carbon atoms, and borated derivatives thereof; the composition having a sulfated ash content less than or equal to 0.5%, as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm as measured according to the ASTM D5185 standard, and a sulfur content less than 0.2% as measured according to the ASTM D5185 standard.
 2. The composition according to claim 1 wherein the ester (c) contains at least one free hydroxyl OH group belonging to the group R, the OH group being located in the alpha, beta or gamma position with respect to the carbon of the CO function of an ester function on which the group R is bound, and/or contains at least one free hydroxyl OH group belonging to the group R′, said OH group being located in the beta, gamma or delta position with respect to the oxygen of the COO group of an ester function on which the group R′ is bound.
 3. The composition according to claim 1, wherein the group R′ of the ester (c) represents a C₁-C₁₀ group.
 4. The composition according to claim 3, wherein p is strictly greater than zero and the group R of the ester (c) represents a C₈-C₂₅ group.
 5. The composition according to claim 4, wherein at least one ester (c) is selected from glycerol monoesters or diesters, preferentially selected from glycerol mono-oleate, glycerol stearate or isostearate and borated derivatives thereof.
 6. The composition according to claim 3, wherein n is an integer comprised between 1 and 4 and the R group of the ester (c) represents a C₁-C₅ group.
 7. The composition according to claim 6, wherein at least one ester (c) is selected from citrates, tartrates, malates, lactates, mandelates, glycolates, hydroxypropionates, hydroxyglutarates or borated derivatives thereof.
 8. The composition according to claim 1, leading to minimum fuel savings as measured on a M111FE test, of at least 2.5% and meeting the ACEA-C1 specifications defined by the European Automobile Manufacturers' Association.
 9. The composition according to claim 1, having a kinematic viscosity at 100° C., as measured by the ASTM D445 standard, comprised between 5.6 and 16.3 cSt.
 10. The composition according to claim 1, with a 5W30 grade according to the SAEJ300 classification.
 11. The composition according to claim 1, with a viscosity index VI larger than or equal to
 130. 12. The composition according to claim 1, wherein the base oil or the mixture of base oils (a) amounts to at least 70% by weight of the composition.
 13. The composition according to claim 1, wherein the base oil or the mixture of base oils (a) comprises: at least 60% by weight, based on the total lubricant weight, of one or more base oils of Group III; and at least 10% by weight, based on the total lubricant weight, of one or more base oils of Group IV.
 14. The composition according to claim 1, comprising a compound (b) selected from the group of heavy PAOs, and a compound (b) selected from the group of polymeric compounds of the FIB type.
 15. The composition according to claim 1, comprising 0.1 to 6% of at least one compound (b) and from 0.1 to 2.5% of at least one compound (c).
 16. The composition according to claim 1, comprising at least one anti-wear compound of the zinc dithiophosphate type, optionally combined with an amine phosphate.
 17. The composition according to claim 1, comprising at least one anti-wear compound of the zinc dithiophosphate type in an amount less than or equal to 1%.
 18. The composition according to claim 1, free of any amine phosphate type additive.
 19. The composition according to claim 1, free of any molybdenum friction modifier additive.
 20. The composition according to claim 1, comprising at least one antioxidant compound, preferably ashless, preferentially of the phenolic or aminated type.
 21. The composition according to claim 1, comprising from 0.01 to 5% of one or more antioxidant additives.
 22. The composition according to claim 1, with a BN, determined according to the ASTM D-2896 standard, less than or equal to 8 milligrams of potash per gram of lubricant.
 23. The composition according to claim 1, comprising between 0 and 3% of a VI-enhancing polymer selected from polymeric esters, olefin copolymers (OCP), homopolymers or copolymers of styrene, butadiene or isoprene, polymethacrylates (PMAs).
 24. A method for making a lubricating composition according to claim 1, by diluting a package of additives comprising at least one compound (b) and at least one compound (c), in a base oil or a mixture of oil bases (a) and wherein a VI-enhancing polymer is optionally added.
 25. The making method according to claim 24, wherein the additive package is diluted so as to amount to 10-30% by weight of the lubricating composition, and wherein the VI-enhancing polymer amounts to 0-3% by weight of the lubricating composition.
 26. A package of additives for a four-stroke engine lubricant with a sulfated ash content less than or equal to 0.5%, as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm, as measured according to ASTM D5185, and a sulfur content less than 0.2%, as measured according to the ASTM D5185 standard, the package comprising: at least one compound (b), a heavy PAO optionally mixed with a PIB; at least one hydroxylated ester (c); and optionally anti-wear and extreme pressure additives, friction modifiers, detergents, antioxidants, detergents either overbased or not, flow point lowering additives, dispersants, anti-foam additives, thickeners, polymers enhancing the viscosity index.
 27. The additive package according to claim 26 comprising: 0.5-30% by weight of at least one compound (b), a heavy PAO optionally mixed with PIB; and 0.5-15% by weight of at least one hydroxylated ester (c).
 28. The use of a composition according to claim 1, as a lubricant for a four-stroke engine.
 29. The use of at least one ester of formula R(OH)_(m) (COOR′(OH)_(p))_(n) wherein m is an integer from 0 to 8, n is an integer from 1 to 8, and p is an integer from 0 to 8, and wherein the sum p+m is strictly greater than zero, R and R′ represent independently of each other a saturated or unsaturated linear or branched hydrocarbon group, optionally substituted with one or more aromatic groups, and including 1 to 30 carbon atoms, and borated derivatives thereof, as a friction modifying agent for the preparation of a lubricating composition for a four-stroke engine with a sulfated ash content less than or equal to 0.5%, as measured according to the ASTM D874 standard, a phosphorus content less than or equal to 500 ppm, as measured according to ASTM D5185, and a sulfur content less than or equal to 0.2%, as measured according to the ASTM D5185 standard. 